The present invention relates to the field of medical lasers. More particularly, the present invention relates to the field of medical lasers for effecting incisions, tissue ablation and coagulation.
A laser beam is formed when a material capable of lasing, such as a solid-state crystal or gas, is excited by incident light energy. In response, ions within the material are pumped to a high energy level and, then, energy is dissipated when the ions return to a ground state. In transitioning from the high energy level to the ground state, the ions each emit a photon in addition to heat energy. The emitted photons have a uniform wavelength (xcex) and eventually form the laser beam.
FIG. 1 schematically illustrates a solid state laser in accordance with the prior art. A cylindrical rod-shaped crystal 10 is disposed between two reflective surfaces 12, 14. The surfaces 12, 14 are aligned parallel to one another and perpendicular to the longitudinal axis of the crystal 10. While light can be emitted from the crystal 10 in various different directions, only a coherent beam of light 16 which travels along the axis of the crystal 10 is reflected between the surfaces 12, 14. The surface 12 has a reflectivity of nearly 100% and, thus, reflects all of the beam 16. The surface 14, however, has a reflectivity of less than 100% and a transmissivity of greater than zero. Thus, a portion 18 of the beam 16 passes through the surface 14. The emitted beam 18 can be utilized for industrial or medical applications. For example, the beam 18 can be directed to a target surface.
A property of a laser beam is that the beam is continually diffracting. This diffraction is evidenced by convergence (narrowing) to a waist or divergence from a waist. FIG. 2 schematically illustrates a laser beam 50 converging to a waist 52 and, then, diverging from the waist 52. The waist 52, whose radius is given as xcfx890, is the narrowest portion of the beam 52. A distance known as the Rayleigh Range (RR), is a distance from the waist 52 that the beam 50 achieves a radius given by the square root of two times xcfx890 (1.414xcfx890). Thus, the Rayleigh Range is a measure of the convergence and divergence of the beam 50. An important relationship which holds for the beam 50 is given by:   RR  =            πω      0      2                      M        2            ⁢      λ      
where M is a constant which characterizes the number of times greater than the diffraction limit is the beam 52. As can be seen from this equation, for a given waist, the Rayleigh Range is longest when M is equal to one. Similarly, for a given Rayleigh Range, the waist is at its most narrow when M is equal to one. The constant M, however, can be greater than one.
As mentioned, when the excited ions of the crystal 10 (FIG. 1) return from an excited state to the ground state, heat is dissipated, in addition to a photon. To avoid excessive heat from building up in the crystal 10, this heat is typically removed by a cooling jacket which surrounds the crystal 10. As a result of removing heat from the crystal 10, the temperature of the crystal is higher in the center than near the outer edges. As shown in FIG. 3a, when the temperature (T) of the crystal 10 is plotted along a vertical axis and distance (X) from the center of the crystal 10 is plotted along a horizontal axis, a parabolic curve results. The refractive index of the crystal 10, however, varies with temperature. As a result, the crystal 10 behaves as a lens (a thermal lens). This thermal lens tends to narrow the beam 16xe2x80x2 at its ends, as schematically shown in FIG. 3b, thereby counter-acting the natural tendency for the beam to diverge, as illustrated in FIG. 2. FIG. 3b illustrates the effects of a thermal lens on the solid state laser of FIG. 1 where the constant M for the beam 16xe2x80x2 is one. When the constant M is equal to one, the beam 16xe2x80x2 is considered to be of mode TEM00. For the emitted beam 18, M is also equal to one.
Under operating conditions where the temperature in the center of the crystal 10 is increased, as shown in FIG. 4a, the effect of the thermal lens is to further narrow the mode TEM00 beam 16xe2x80x3. FIG. 4b schematically illustrates the effects of increasing temperatures in the solid state laser of FIG. 1. Note that in FIG. 4b, the TEM00 beam 16xe2x80x3 does not pass through the outermost portions of the crystal 10. Light emitted from these regions, forms a beam 20 of another mode (e.g., TEM01). The constant M for the beam 20 of mode TEM01 is 1.4. A beam 22 which emits from the laser shown in FIG. 4b has a constant M which is between 1 and 1.4. Further increases in the strength of the thermal lens results in increasing values for the constant M of the beam 22.
Lasers are used in medical procedures to rejuvenate, restore and resurface skin damaged due to many causes including prolonged exposure to the sun. Laser energy is delivered to the surface of the skin in a controlled pattern in order to ablate or burn away layers of the skin. As the layers of skin grow back within the area of skin exposed to the laser, the skin is effectively resurfaced. To avoid excessive bleeding, it is important that a zone of thermal necrosis or coagulation is formed within the newly exposed tissue. In addition, lasers are used for vision correction by reshaping the lens in the eye. Lasers are also used for forming incisions by ablating a narrow band of tissue.
For each of these functions, the intensity and distribution of the laser beam, its wavelength, and the duration of exposure, all must be understood and/or appropriately controlled so that the desired results are achieved. Advances in laser apparatus have been directed to the difficulty in controlling these factors.
U.S. Pat. No. 4,791,927 discloses a dual-wavelength laser scalpel. A short wavelength blue light cuts the target tissue and a longer-wavelength red light cauterizes. The two wavelengths are formed as the fundamental frequency and second harmonic of a single laser source.
U.S. Pat. No. 5,651,784 discloses a rotatable aperture apparatus and methods for selective photoablation of surfaces. The intensity distribution of a beam of radiation is modified by inserting a rotatable mask into the beam. The mask is formed with one or more apertures that have a geometric spiral shape originating substantially from the center of rotation of the mask. A beam of radiation incident on the rotating mask is transmitted therethrough with intensity that varies as a function of radial position with respect to the rotation point.
U.S. Pat. No. 4,887,019 discloses a device for the generation of a laser beam spot of adjustable size on an object, in particular, in the human eye. A focusing device projects the laser beam onto the eye with a small focusing spot and a large aperture cone. A deflector device moves the focusing spot over the desired beam spot in a predetermined scanning pattern.
U.S. Pat. No. 4,941,093 discloses surface erosion using lasers for eroding a surface, such as a patient""s cornea. A laser beam exits an optical system and is incident on the surface. An iris is placed in the beam between the optical system and the surface which can be opened while pulsing the beam so as to erode the center of the surface to a greater extent than the surrounding area. However, this patent teaches that iris diaphragms are undesirable because the shape of the opening can change along with its size. Thus, an alternative system is disclosed in U.S. Pat. No. 4,941,093 in which a beam shaping stop is placed between the optical system and the surface. The beam shaping stop is arranged to move along the beam axis in a direction of convergence or divergence of the beam.
What is needed is a method and apparatus for controlling the delivery of laser energy. What is further needed is a method and apparatus for controlling the delivery of laser energy for performing medical procedures.
The invention is a selective aperture for a laser delivery system for providing incision, ablation and coagulation. A laser crystal is disposed between two reflective surfaces for forming a laser beam. Preferably, the laser is an erbium YAG laser. An aperture member is positioned between the laser crystal and one of the reflective surfaces. The aperture member includes a substantially circular aperture for passing the laser beam. The size of the aperture is selectively adjustable. As a result, the waist and Rayleigh Range of the beam can be altered to suit the particular procedure being performed. Preferably, the aperture member has a plurality of apertures of various different sizes. In which case, the aperture member is rotatable about an axis of rotation. The axis of rotation is parallel to the longitudinal axis of the laser crystal. By appropriately rotating the aperture member, a selected one of the plurality of apertures is positioned to pass the laser beam. A stepper motor and flexible shaft can be utilized for rotating the aperture member. Preferably, at least one of the apertures is surrounded by a beveled portion of the rotatable member. The beveled portion is cone shaped and can have a cone angle of at least 100 degrees.
According to an aspect of the invention, an articulated arm is provided along with one or more refocussing optics for refocussing the laser beam as it travels through the arm. According to another aspect, a second laser source is provided along with a galvanometer for directing each of two laser beams to a surface to be treated. Such an arrangement provides exceptional versatility and control over the beam emitted. For example, one of the laser sources can be configured for a small waist size for forming an incision, while the other can be configured for a larger waist size for cauterizing the incision. By switching back and forth between pulses from each of the two lasers an incision can be cauterized concurrently as it is being made. Alternately, one of the lasers can be configured for ablating skin while the other is configured for performing coagulation. Additionally, one laser source could be configured for forming a precise incision and another for large area tissue ablation.
According to an another aspect of the invention, the aperture for a single laser source is rapidly changed during the performance of a procedure (i.e. xe2x80x9con-the-flyxe2x80x99). For example, by rapidly switching between forming an incision and cauterizing as the laser beam is pulsed, the laser system can cauterize an incision while it is being made. Although the functions are performed successively, by rapidly switching back and forth between appropriate apertures (time-division-multiplexing) in relation to the speed at which the incision is being made, the functions are effectively performed concurrently.